1. Field of the Invention
This invention relates to a shock absorber adapted to absorb impact energy therein, such as a load imparted to a bumper of an automobile, for example, when the automobile collides with something, and prevent or suppress the transmission of the impact energy to a vehicle body member. More particularly, the invention relates to a shock absorber adapted to absorb impact energy therein by converting the impact energy into deformation energy for plastically deforming a tube portion thereof.
2. Description of the Related Art
In order to prevent or suppress the transmission of impact energy, a load imparted to a bumper of an automobile to a vehicle body member, a shock absorber adapted to absorb the impact energy therein is usually provided between the bumper and a side member. Such shock absorbers include a cylinder type shock absorber, and a plastic deformation type shock absorber. Although the latter plastic deformation type shock absorber has a simple construction, and light weight as compared with the cylinder type shock absorber, the plastic deformation type shock absorber has the advantage of displaying an excellent impact energy absorbing performance and being capable of changing the design thereof flexibly and easily in accordance with a vehicle weight difference. For these reasons, the plastic deformation type shock absorber is used in many cases as seen in JP-A-2001-047952, JP-A-2001-138841, U.S. Pat. No. 6,293,587 and USP Laid-Open 2003/0034661.
As seen in FIG. 9, a plastic deformation type shock absorber has a body as a main element (which will hereinafter be referred to as a multi-diameter stepped tube or a two-stage tube) of a stepped tube structure including a smaller-diameter tube portion 2 and a larger-diameter tube portion 3 which are joined integrally via a stepped portion 7. The smaller-diameter tube portion 2 is connected to a bumper 10, and the larger-diameter tube portion 3 to a side member 8. The impact energy transmitted from the bumper 10 works to compress the two-stage tube 1 as a whole in the longitudinal direction of a vehicle, and the impact energy is absorbed as the energy occurring at this time for plastically deforming the two-stage tube 1. During this time, the two-stage tube 1 is locally supported on an intersection of a rear end peripheral edge 11 of the larger-diameter tube portion 3 and a frame of the side member 8, so that a load is necessarily imparted to the intersection in a concentrated manner.
In order to sufficiently display the impact energy absorbing function of the plastic deformation type shock absorber, it is necessary that the multi-diameter stepped tube as a whole be compressed uniformly without causing the vehicle body member to be deformed in an impact absorbing operation. However, when a local load is imparted to the intersection of the two-stage tube 1 and side member 8 which supports the two-stage tube 1 thereon, as mentioned above, the vehicle body member becomes liable to be relatively deformed due to the concentrated impact energy, so that it becomes unable to make the best use of the performance which the shock absorber possesses. When, in a shock absorbing operation, upper and lower edges of a mounting part 9 provided at a front end of the side member 8 are bent as seen in, for example, FIG. 10 in the direction opposite to the impact imparting direction at an end surface as a boundary surface of the larger-diameter tube portion, the side member 8 is collapsed or deformed. As a result, the function of the shock absorber of preventing or suppressing the transmission of a load to the vehicle body members is restricted.
Improving the rigidity of a front end of the side member connected to a rear end of the shock absorber including increasing the thickness of, for example, a plate constituting the mounting part 9 is conceived as a method of solving such problems. However, such measures cause the cost of materials to increase, and improving the strength of a part of the structure gives rise to a problem of losing a balance of the strength of the vehicle body member as a whole. Under the circumstances, a shock absorber capable of preventing the collapse or deformation of vehicle body member ascribed to the local application of a load to the shock absorber, without being influenced by the arrangement or construction of the vehicle body members.